This invention relates to a phase error detection apparatus and method for detecting an error which occurs with a phase difference between a magnetic domain wall displacement which occurs at a leading end of an isothermal region formed on a recording medium and another magnetic domain wall displacement which occurs at a trailing end of the isothermal region, and a laser power control apparatus and method for controlling the laser power using the phase error detection apparatus and method, as well as a magneto-optical recording and/or playback apparatus for playing back an information signal from a magneto-optical recording medium using the laser power control apparatus and method.
Conventionally, various optical disks have been proposed as a recording medium for an information signal, and as an optical disk which allows rewriting of an information signal, a magneto-optical (MO) disk and a phase change (PC) disk have been proposed. Investigations have been and are being performed to allow higher density recording of such magneto-optical disk and phase change disk. However, it is considered to be difficult to read a mark smaller than a reading light spot.
However, for the magneto-optical disk, a super-resolution technique for masking marks which are not an object of playback to eliminate intersymbol interference while playing back only marks of an object of playback has been developed.
Further, a magnetic domain expansion and playback system (MAMMOS: Magnetic AMplifying Magneto-Optical System) and a magnetic domain wall displacement detection system (DWDD) have been proposed recently for playback of a magneto-optical disk recorded in a high density.
Both of the magnetic domain expansion and playback and the magnetic domain wall displacement detection expand the magnetic domain of a mark smaller than a light spot for playback through a distribution of heat generated by the light spot to read the mark. When the magnetic domain expansion and playback and the magnetic domain wall displacement detection are compared with each other, the latter can detect an edge of a mark clearly while the former can detect the position of a mark with certainty. Accordingly, the magnetic domain wall displacement detection is suitable to play back an optical disk which adopts xe2x80x9cmark edge recordingxe2x80x9d.
The principle of playback by the magnetic domain wall displacement detection is described using an isolated mark. In particular, according to the magnetic domain wall displacement detection, presence of a mark is detected making use of the fact that, when a magnetic domain wall of an expansion layer comes to a leading end of an isothermal region in which the temperature generated by a laser beam is higher than the Curie temperature, the magnetic domain wall moves quickly to a maximum temperature portion.
Where such magnetic domain wall displacement detection as described above is used to play back a magneto-optical disk, also when the magnetic domain wall comes to the trailing end of the isothermal region, the magnetic domain wall moves quickly to a maximum temperature portion. Therefore, a read signal (MO signal) by the magnetic domain wall displacement detection includes not only a data signal but also a ghost signal having an equal length to that of the data signal and having a level lower then that of the data signal after a delay of time of xe2x80x9cisothermal region length÷linear velocityxe2x80x9d from the data signal. If the ghost signal is played back together with the data signal, then it is difficult to play hack an information signal recorded in a high density and also it is difficult to play back a clock signal.
Thus, a technique for canceling a ghost has been proposed in Japanese Patent Laid-open No. Hei 11-296928 by the assignee of the present application. According to the technique, when the magnetic domain wall displacement detection is used to play back a magneto-optical disk, an external clock system is used to play hack a clock signal stably, and the clock signal is used to convert the level of a RF signal read from the magneto-optical disk into a digital signal so that ghost signal processing can be performed by digital signal processing to allow the data signal to be played back correctly.
However, in order to obtain a correct data signal using the technique disclosed in the document specified as above, a process for separating the data signal and the ghost signal from each other is required.
It is an object of the present invention to provide a phase error detection apparatus and method which can detect not only an error which occurs with a phase difference of a ghost signal from a data signal in a unit of a clock but also an error of the phase difference which appears in a shorter period than that of the clock.
It is another object of the present invention to provide a laser power control apparatus and method by which the playback laser power can be kept at an optimum level based not only on an error which occurs with a phase difference of a ghost signal from a data signal in a unit of a clock but also an error of the phase difference which appears in a shorter period than that of the clock.
It is a further object of the present invention to provide a magneto-optical recording and/or playback apparatus by which a data signal can be played back in a high S/N ratio from a magneto-optical disk making use of a ghost signal by the magnetic domain wall displacement detection while keeping the playback laser power at an optimum level.
In order to attain the objects described above, according to the present invention, the laser power is first controlled so that the time delay of a ghost may be a fixed multiple of a data detection clock. Then, the laser power is adjusted so that the time delay may be a fixed multiple of the clock nearest to the phase delay amount set in advance of a ghost which appears in the isothermal region with a laser power with which an optimum signal characteristic is obtained. In this instance, the level of the playback signal is detected as a sum of results of displacement of both of a magnetic wall at a leading end of the isothermal region and another magnetic wall at a trailing end of the isothermal region after the fixed multiple at the time.
In particular, according to the present invention, there is provided a phase error detection apparatus, including detection means for detecting, based on a reference pattern including a record mark which appears repetitively at predetermined bit intervals, an error appearing in a phase difference between a magnetic domain wall displacement which occurs with a leading end of an isothermal region formed on a recording medium by irradiation of a laser beam and another magnetic domain wall displacement which occurs with a trailing end of the isothermal region.
The phase error detection apparatus may detect an error of the phase difference which occurs in a unit of the clock signal or an error of the phase difference which appears in a unit of time shorter than a clock signal.
According to another aspect of the present invention, there is provided a phase error detection method for detecting an error appearing in a phase difference between a magnetic domain wall displacement which occurs with a leading end of an isothermal region formed on a recording medium by irradiation of a laser beam and another magnetic domain wall displacement which occurs with a trailing end of the isothermal region, including a step of detecting a reference pattern including a record mark which appears repetitively at predetermined bit intervals, and a step of detecting an error of the phase difference which appears in a unit of time shorter than a clock signal based on the detected reference pattern.
With the phase error detection apparatus and method, not only a phase difference in a unit of a clock of a ghost signal from a data signal but also another phase difference which appears in a time shorter than the clock can be detected.
According to a further aspect of the present invention, there is provided a laser power control apparatus, including phase error detection means for detecting, based on a reference pattern including a record mark which appears repetitively at predetermined bit number intervals, an error appearing in a phase difference between a magnetic domain wall displacement which occurs with a leading end of an isothermal region formed on a recording medium by irradiation of a playback laser beam and another magnetic domain wall displacement which occurs with a trailing end of the isothermal region, amplitude detection means for detecting an amplitude of the predetermined bit number period of the reference pattern based on a clock signal, and control means for controlling a playback laser power based on a result of the detection from the phase error detection means and a result of the detection from the amplitude detection means.
The phase error detection means detects an error of the phase difference which occurs in a unit of the clock signal. The control means may discriminate an error of the phase difference which occurs in a unit of the clock signal based on the amount of jitters of the clock signal. In this instance, the control means may set a playback laser power when the amount of jitters exhibits a bottom and a 5 T amplitude detected by the amplitude detection means exhibits a peak.
The phase error detection means detects an error of the phase difference which occurs in a unit of time shorter than the clock signal. In this instance, the control means may discriminate the error of the phase difference which occurs in a unit of the clock signal and discriminate also the error of the phase difference which occurs in a unit of time shorter than the clock signal. Further, the control means may set the playback laser power when the amount of jitters exhibits a bottom and a 5 T amplitude detected by the amplitude detection means exhibits a peak and besides the error of the phase difference exhibits a zero cross.
According to a still further aspect of the present invention, there is provided a laser power control method, including a step of detecting, based on a reference pattern including a record mark which appears repetitively at predetermined bit number intervals, an error appearing in a phase difference between a magnetic domain wall displacement which occurs with a leading end of an isothermal region formed on a recording medium by irradiation of a playback laser beam and another magnetic domain wall displacement which occurs with a trailing end of the isothermal region, a step of detecting an amplitude of the predetermined bit number period of the reference pattern based on a clock signal, and a step of setting a playback laser power based on a result of the detection by the step of detecting a phase error and a result of the detection by the step of detecting amplitude.
With the laser power control apparatus and method, the playback laser power can be kept at an optimum level using not only a phase difference in a unit of a clock of a ghost signal from a data signal but also another phase difference which appears in a time shorter than the clock can be detected.
According to a yet further aspect of the present invention, there is provided a magneto-optical recording and/or playback apparatus, including optical pickup means including a laser light source for emitting a laser beam, the optical pickup means being operable to irradiate the laser beam emitted from the laser light source upon a playback layer laminated on a record layer of a magneto-optical recording medium to read an information signal recorded on the record layer of the magneto-optical recording medium by magnetic domain expansions which appear in an isothermal region of the magneto-optical recording medium which appears in response to the laser beam and output a corresponding received light amount signal, playback signal generation means for generating a sum component of the magneto-optical signal and a difference component of the magneto-optical signal based on the received light amount signal outputted from the optical pickup means, and laser power control means for controlling a playback laser power to be supplied to the laser light source based on the sum component and the difference component of the magneto-optical signal supplied thereto from the playback signal generation means.
The laser power control means may include phase error detection means for detecting, based on a reference pattern including a record mark which appears repetitively at predetermined bit number intervals, an error appearing in a phase difference between a magnetic domain wall displacement which occurs with a leading end of the isothermal region formed on the recording medium by irradiation of the playback laser beam and another magnetic domain wall displacement which occurs with a trailing end of the isothermal region, amplitude detection means for detecting an amplitude of the predetermined number period of the reference pattern based on a clock signal, and control means for controlling a playback laser power based on a result of the detection from the phase error detection means and a result of the detection from the amplitude detection means.
With the magneto-optical recording and/or playback apparatus, a data signal can be played back at a high S/N ratio from a magneto-optical disk making use of a ghost signal by magnetic wall displacement detection while the playback laser power is kept at an optimum level.